A method of connecting a branch pipe to an existing main pipe is disclosed. The method includes the steps of: connecting and communicating a branch-pipe joint having a branching pipe portion to an existing main pipe; connecting a branch pipe to the branch-pipe joint; and communicating the branch pipe with the existing main pipe through the branch-pipe joint, with all these steps being effected while outflow of fluid from inside the main pipe being prevented. In the above method, the branch pipe is connected with a connecting pipe portion which in turn is connected with the branching pipe portion via a flange joint including a pair of flanges. A shutter plate is inserted between the pair of flanges of the flange joint and communicating the connecting pipe portion with the branching pipe portion thus preventing outflow of the fluid from the branching pipe portion. The flange joint is loosened while the connecting pipe portion is kept connected with the branch pipe so as to withdraw the shutter plate from between the flanges to allow communication between the branch pipe and the branching pipe portion. The flange joint is then tightened and the connecting pipe portion the branching pipe portion are connected and communicated with each other.